Currently, there is a trend in the wind energy sector toward higher power rating of wind turbines in order to reduce the cost of energy. This causes a subsequent increase in weight and dimensions of most of the components comprised in the wind turbine, including the rotor. Another trend for increasing energy capture has been to design taller towers to take advantage of greater wind energy at higher levels above ground, most of the designs nowadays being 100 m high and taller.
Particularly relevant for the assembly and maintenance costs are tower height, rotor diameter and weight of hub and blades, due to their direct impact on crane capacity and cost of the same, being the most important the first of them. Tower height determines the system and process required for assembling components relatively heavy and/or bulky, such as the hub or the blades: an increase in height implies bigger bending moments that have to be stood by the cranes, and that make necessary the use of higher capacity cranes than the actual weight being lifted and whose use can be really expensive.
In the specific case of operations for assembling blades, there are three procedures most commonly used:
Preassembling the hub and the blades at ground level and lifting the preassembled system to the top of the nacelle;
Lifting and assembling each blade one by one in a horizontal position and connecting them to the hub on top of the tower. In order to lift the blade in such a position and avoid it from turning and collapsing, this process requires either two cranes connected to the blade in both sides of it, or a single crane making use of a special tool to compensate possible variations of the center of gravity of the blade in relation to the position of the lifting point; and
Lifting and assembling each blade one by one in a vertical position and connecting them to the hub on top of the tower. It usually comprises a hoisting system and a plurality of pulleys installed within the hub and nacelle, in order to avoid the use of a big crane.
Among the last of the listed procedures there are several alternatives in the state of the art:
US Patent Application Publication US 2009/0058096 (Wobben) discloses a method of mounting or dismantling a rotor blade of a wind power installation without using a crane. A cable is stretched between a part in the outside surface of the hub of the wind power installation and the bottom region of the wind power installation. The rotor blade is moved along the cable upwardly upon mounting or downwardly upon dismounting the blade.
U.S. Pat. No. 8,083,212B2 discloses a method of mounting a rotor blade, which allows the blade to be mounted and removed without using multiple construction machines. The suspension device has a pulley inside the hub, a holding section inside or outside a blade, and a winch. The pulley leads a suspension wire from the rotor hub, on which the blade is to be mounted, to the holding section fixed at the root end and inside the blade. The holding section holds the mounting end side of the blade such that a line connecting the center of gravity of the blade and a connection section to which the suspension wire is connected intersects with the longitudinal axis of the blade. The winch is placed on the ground and raises and lowers the holding section/blade by pulling in and paying out the suspension rope.
WO2011064659A2 discloses a system for assembling a rotor blade comprising a modular truss system including at least one pulley and two mounting beams, each mounting beam being detachably connected to a hub connection flange, thereby positioning the pulley beam above a further hub connection flange and a blade pulley system including at least one pulley and being detachably connected to the blade connector portion of a rotor blade. To avoid any damage to the hub connection flange or the pitch bearing, they are aligned before they come in contact with each other by mean of a plurality of flexible cables, each of them connected to the outer race of the pitch bearing of the blade being assembled, passing through holes in the respective flange of the hub and further connected to a hoist arranged in the hub or the nacelle.
The above-described publications have the disadvantage of comprising elements of the lifting system (i.e. pulleys, winches, cables, etc.) inside the hub and, therefore, their position for mounting one blade is not proper for the assembly of another one: these elements have to be adequately mounted once per blade assembled; that is thrice for a conventional rotor. This operation is time consuming and increments the cost of the whole method. Moreover, those elements have to be disassembled and lowered to the ground once the operation has finished, and so, they have to be moved to an access door provided in the rear of the nacelle or in the front of the hub.
Moreover and even more important, they do not provide an efficient and robust system for positioning and guiding the blade into engagement with the hub connection flange, this operation being crucial for preventing damage from being infringed to any element.